Distributed systems concepts and design

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Distributed Systems, Edition 3: Chapter1 Solutions 1 Last updated: 18 September 2000 3:53 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 Chapter 1 Exercise Solutions 1.1 Give five types of hardware resource and five types of data or software resource that can usefully be shared. Give examples of their sharing as it occurs in distributed systems. 1.1 Ans. Hardware: CPU: compute server (executes processor-intensive applications for clients), remote object server (executes methods on behalf of clients), worm program (shares cpu capacity of desktop machine with the local user). Most other servers, such as file servers, do some computation for their clients, hence their cpu is a shared resource. memory: cache server (holds recently-accessed web pages in its RAM, for faster access by other local computers) disk: file server, virtual disk server (see Chapter 8), video on demand server (see Chapter 15). screen: Network window systems, such as X-11, allow processes in remote computers to update the content of windows. printer: networked printers accept print jobs from many computers. managing them with a queuing system. network capacity: packet transmission enables many simultaneous communication channels (streams of data) to be transmitted on the same circuits. Data/software: web page: web servers enable multiple clients to share read-only page content (usually stored in a file, but sometimes generated on-the-fly). file: file servers enable multiple clients to share read-write files. Conflicting updates may result in inconsistent results. Most useful for files that change infrequently, such as software binaries. object: possibilities for software objects are limitless. E.g. shared whiteboard, shared diary, room booking system, etc. database: databases are intended to record the definitive state of some related sets of data. They have been shared ever since multi-user computers appeared. They include techniques to manage concurrent updates. newsgroup content: The netnews system makes read-only copies of the recently-posted news items available to clients throughout the Internet. A copy of newsgroup content is maintained at each netnews server that is an approximate replica of those at other servers. Each server makes its data available to multiple clients. video/audio stream: Servers can store entire videos on disk and deliver them at playback speed to multiple clients simultaneously. exclusive lock: a system-level object provided by a lock server, enabling several clients to coordinate their use of a resource (such as printer that does not include a queuing scheme). Distributed Systems: Concepts and Design Edition 3 By George Coulouris, Jean Dollimore and Tim Kindberg Addison-Wesley, ©Pearson Education 2001 Distributed Systems, Edition 3: Chapter1 Solutions 2 Last updated: 18 September 2000 3:53 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 1.2 How might the clocks in two computers that are linked by a local network be synchronized without reference to an external time source? What factors limit the accuracy of the procedure you have described? How could the clocks in a large number of computers connected by the Internet be synchronized? Discuss the accuracy of that procedure. 1.2 Ans. Several time synchronization protocols are described in Section 10.3. One of these is Cristian’s protocol. Briefly, the round trip time t to send a message and a reply between computer A and computer B is measured by repeated tests; then computer A sends its clock setting T to computer B. B sets its clock to T + t /2. The setting can be refined by repetition. The procedure is subject to inaccuracy because of contention for the use of the local network from other computers and delays in the processing the messages in the operating systems of A and B. For a local network, the accuracy is probably within 1 ms. For a large number of computers, one computer should be nominated to act as the time server and it should carry out Cristian’s protocol with all of them. The protocol can be initiated by each in turn. Additional inaccuracies arise in the Internet because messages are delayed as they pass through switches in wider area networks. For a wide area network the accuracy is probably within 5-10 ms. These answers do not take into account the need for fault-tolerance. See Chapter 10 for further details. 1.3 A user arrives at a railway station that she has never visited before, carrying a PDA that is capable of wireless networking. Suggest how the user could be provided with information about the local services and amenities at that station, without entering the station’s name or attributes. What technical challenges must be overcome? 1.3 Ans. The user must be able to acquire the address of locally relevant information as automatically as possible. One method is for the local wireless network to provide the URL of web pages about the locality over a local wireless network. For this to work: (1) the user must run a program on her device that listens for these URLs, and which gives the user sufficient control that she is not swamped by unwanted URLs of the places she passes through; and (2) the means of propagating the URL (e.g. infrared or an 802.11 wireless LAN) should have a reach that corresponds to the physical spread of the place itself. 1.4 What are the advantages and disadvantages of HTML, URLs and HTTP as core technologies for information browsing? Are any of these technologies suitable as a basis for client-server computing in general? 1.4 Ans. HTML is a relatively straightforward language to parse and render but it confuses presentation with the underlying data that is being presented. URLs are efficient resource locators but they are not sufficiently rich as resource links. For example, they may point at a resource that has been relocated or destroyed; their granularity (a whole resource) is too coarse- grained for many purposes. HTTP is a simple protocol that can be implemented with a small footprint, and which can be put to use in many types of content transfer and other types of service. Its verbosity (HTML messages tend to contain many strings) makes it inefficient for passing small amounts of data. HTTP and URLs are acceptable as a basis for client-server computing except that (a) there is no strong type- checking (web services operate by-value type checking without compiler support), (b) there is the inefficiency that we have mentioned. 1.5 Use the World Wide Web as an example to illustrate the concept of resource sharing, client and server. Resources in the World Wide Web and other services are named by URLs. What do the initials Distributed Systems, Edition 3: Chapter1 Solutions 3 Last updated: 18 September 2000 3:53 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 URL denote? Give examples of three different sorts of web resources that can be named by URLs. 1.5 Ans. Web Pages are examples of resources that are shared. These resources are managed by Web servers. Client-server architecture. The Web Browser is a client program (e.g. Netscape) that runs on the user's computer. The Web server accesses local files containing the Web pages and then supplies them to client browser processes. URL - Uniform Resource Locator (3 of the following a file or a image, movies, sound, anything that can be rendered, a query to a database or to a search engine. 1.6 Give an example of a URL. List the three main components of a URL, stating how their boundaries are denoted and illustrating each one from your example. To what extent is a URL location transparent? 1.6 Ans. http://www.dcs.qmw.ac.uk/research/distrib/index.html • The protocol to use. the part before the colon, in the example the protocol to use is http ("HyperText Transport Protocol"). • The part between // and / is the Domain name of the Web server host www.dcs.qmw.ac.uk. • The remainder refers to information on that host - named within the top level directory used by that Web server research/distrib/book.html. The hostname www is location independent so we have location transparency in that the address of a particular computer is not included. Therefore the organisation may move the Web service to another computer. But if the responsibility for providing a WWW-based information service moves to another organisation, the URL would need to be changed. 1.7 A server program written in one language (for example C++) provides the implementation of a BLOB object that is intended to be accessed by clients that may be written in a different language (for example Java). The client and server computers may have different hardware, but all of them are attached to an internet. Describe the problems due to each of the five aspects of heterogeneity that need to be solved to make it possible for a client object to invoke a method on the server object. 1.7 Ans. As the computers are attached to an internet, we can assume that Internet protocols deal with differences in networks. But the computers may have different hardware - therefore we have to deal with differences of representation of data items in request and reply messages from clients to objects. A common standard will be defined for each type of data item that must be transmitted between the object and its clients. The computers may run different operating systems, therefore we need to deal with different operations to send and receive messages or to express invocations. Thus at the Java/C++ level a common operation would be used which will be translated to the particular operation according to the operating system it runs on. We have two different programming languages C++ and Java, they use different representations for data structures such as strings, arrays, records. A common standard will be defined for each type of data structure that must be transmitted between the object and its clients and a way of translating between that data structure and each of the languages. We may have different implementors, e.g. one for C++ and the other for Java. They will need to agree on the common standards mentioned above and to document them. Distributed Systems, Edition 3: Chapter1 Solutions 4 Last updated: 18 September 2000 3:53 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 1.8 An open distributed system allows new resource sharing services such as the BLOB object in Exercise 1.7 to be added and accessed by a variety of client programs. Discuss in the context of this example, to what extent the needs of openness differ from those of heterogeneity. 1.8 Ans. To add the BLOB object to an existing open distributed system, the standards mentioned in the answer to Exercise 1.7 must already have been agreed for the distributed system To list them again: • the distributed system uses a common set of communication protocols (probably Internet protocols). • it uses an defined standard for representing data items (to deal with heterogeneity of hardware). • It uses a common standard for message passing operations (or for invocations). • It uses a language independent standard for representing data structures. But for the open distributed system the standards must have been agreed and documented before the BLOB object was implemented. The implementors must conform to those standards. In addition, the interface to the BLOB object must be published so that when it is added to the system, both existing and new clients will be able to access it. The publication of the standards allows parts of the system to be implemented by different vendors and to work together. 1.9 Suppose that the operations of the BLOB object are separated into two categories – public operations that are available to all users and protected operations that are available only to certain named users. State all of the problems involved in ensuring that only the named users can use a protected operation. Supposing that access to a protected operation provides information that should not be revealed to all users, what further problems arise? 1.9 Ans. Each request to access a protected operation must include the identity of the user making the request. The problems are: • defining the identities of the users. Using these identities in the list of users who are allowed to access the protected operations at the implementation of the BLOB object. And in the request messages. • ensuring that the identity supplied comes from the user it purports to be and not some other user pretending to be that user. • preventing other users from replaying or tampering with the request messages of legitimate users. Further problems. • the information returned as the result of a protected operation must be hidden from unauthorised users. This means that the messages containing the information must be encrypted in case they are intercepted by unauthorised users. 1.10 The INFO service manages a potentially very large set of resources, each of which can be accessed by users throughout the Internet by means of a key (a string name). Discuss an approach to the design of the names of the resources that achieves the minimum loss of performance as the number of resources in the service increases. Suggest how the INFO service can be implemented so as to avoid performance bottlenecks when the number of users becomes very large. 1.10 Ans. Algorithms that use hierarchic structures scale better than those that use linear structures. Therefore the solution should suggest a hierarchic naming scheme. e.g. that each resource has an name of the form ’A.B.C’ etc. where the time taken is O(log n) where there are n resources in the system. To allow for large numbers of users, the resources are partitioned amongst several servers, e.g. names starting with A at server 1, with B at server 2 and so forth. There could be more than one level of partitioning as in DNS. To avoid performance bottlenecks the algorithm for looking up a name must be decentralised. That is, the same server must not be involved in looking up every name. (A centralised solution would use a single root server that holds a location database that maps parts of the information onto particular servers). Some replication is required to avoid such centralisation. For example: i) the location database might be replicated Distributed Systems, Edition 3: Chapter1 Solutions 5 Last updated: 18 September 2000 3:53 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 at multiple root servers or ii) the location database might be replicated in every server. In both cases, different clients must access different servers (e.g. local ones or randomly). 1.11 List the three main software components that may fail when a client process invokes a method in a server object, giving an example of a failure in each case. To what extent are these failures independent of one another? Suggest how the components can be made to tolerate one another’s failures. 1.11 Ans. The three main software components that may fail are: • the client process e.g. it may crash • the server process e.g. the process may crash • the communication software e.g. a message may fail to arrive The failures are generally caused independently of one another. Examples of dependent failures: • if the loss of a message causes the client or server process to crash. (The crashing of a server would cause a client to perceive that a reply message is missing and might indirectly cause it to fail). • if clients crashing cause servers problems. • if the crash of a process causes a failures in the communication software. Both processes should be able to tolerate missing messages. The client must tolerate a missing reply message after it has sent an invocation request message. Instead of making the user wait forever for the reply, a client process could use a timeout and then tell the user it has not been able to contact the server. A simple server just waits for request messages, executes invocations and sends replies. It should be absolutely immune to lost messages. But if a server stores information about its clients it might eventually fail if clients crash without informing the server (so that it can remove redundant information). (See stateless servers in chapter 4/5/8). The communication software should be designed to tolerate crashes in the communicating processes. For example, the failure of one process should not cause problems in the communication between the surviving processes. 1.12 A server process maintains a shared information object such as the BLOB object of Exercise 1.7. Give arguments for and against allowing the client requests to be executed concurrently by the server. In the case that they are executed concurrently, give an example of possible ‘interference’ that can occur between the operations of different clients. Suggest how such interference may be prevented. 1.12 Ans. For concurrent executions - more throughput in the server (particularly if the server has to access a disk or another service) Against - problems of interference between concurrent operations Example: Client A’s thread reads value of variable X Client B’s thread reads value of variable X Client A’s thread adds 1 to its value and stores the result in X Client B’s thread subtracts 1 from its value and stores the result in X Result: X := X-1; imagine that X is the balance of a bank account, and clients A and B are implementing credit and debit transactions, and you can see immediately that the result is incorrect. To overcome interference use some form of concurrency control. For example, for a Java server use synchronized operations such as credit and debit. Distributed Systems, Edition 3: Chapter1 Solutions 6 Last updated: 18 September 2000 3:53 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 1.13 A service is implemented by several servers. Explain why resources might be transferred between them. Would it be satisfactory for clients to multicast all requests to the group of servers as a way of achieving mobility transparency for clients? 1.13 Ans. Migration of resources (information objects) is performed: to reduce communication delays (place objects in a server that is on the same local network as their most frequent users); to balance the load of processing and or storage utilisation between different servers. If all servers receive all requests, the communication load on the network is much increased and servers must do unnecessary work filtering out requests for objects that they do not hold. Distributed Systems, Edition 3: Chapter 2 Solutions 1 Last updated: 9 October 2000 6:12 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 Chapter 2 Exercise Solutions 2.1 Describe and illustrate the client-server architecture of one or more major Internet applications (for example the Web, email or netnews). 2.1 Ans. Web: Browsers are clients of Domain Name Servers (DNS) and web servers (HTTP). Some intranets are configured to interpose a Proxy server. Proxy servers fulfil several purposes – when they are located at the same site as the client, they reduce network delays and network traffic. When they are at the same site as the server, they form a security checkpoint (see pp. 107 and 271) and they can reduce load on the server. N.B. DNS servers are also involved in all of the application architectures described below, but they ore omitted from the discussion for clarity. Email: Sending messages: User Agent (the user’s mail composing program) is a client of a local SMTP server and passes each outgoing message to the SMTP server for delivery. The local SMTP server uses mail routing tables to determine a route for each message and then forwards the message to the next SMTP server on the chosen route. Each SMTP server similarly processes and forwards each incoming message unless the domain name in the message address matches the local domain. In the latter case, it attempts to deliver the message to local recipient by storing it in a mailbox file on a local disk or file server. Reading messages: User Agent (the user’s mail reading program) is either a client of the local file server or a client of a mail delivery server such as a POP or IMAP server. In the former case, the User Agent reads messages directly form the mailbox file in which they were placed during the message delivery. (Exampes of such user agents are the UNIX mail and pine commands.) In the latter case, the User Agent requests information about the contents of the user’s mailbox file from a POP or IMAP server and receives messages from those servers for presentation to the user. POP and IMAP are protocols specifically designed to support mail access over wide areas and slow network connections, so a user can continue to access her home mailbox while travelling. Distributed Systems: Concepts and Design Edition 3 By George Coulouris, Jean Dollimore and Tim Kindberg Addison-Wesley, ©Pearson Education 2001. Proxy server Browser Proxy Web server Web server server Browser DNS server DNS server HTTP HTTP HTTP DNS Distributed Systems, Edition 3: Chapter 2 Solutions 2 Last updated: 9 October 2000 6:12 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 Netnews: Posting news articles: User Agent (the user’s news composing program) is a client of a local NNTP server and passes each outgoing article to the NNTP server for delivery. Each article is assigned a unique identifier. Each NNTP server holds a list of other NNTP servers for which it is a newsfeed – they are registered to receive articles from it. It periodically contacts each of the registered servers, delivers any new articles to them and requests any that they have which it has not (using the articles’ unique id’s to determine which they are). To ensure delivery of every article to every Netnews destination, there must be a path of newsfeed connections from that reaches every NNTP server. Browsing/reading articles: User Agent (the user’s news reading program) is a client of a local NNTP server. The User Agent requests updates for all of the newsgroups to which the user subscribes and presents them to the user. IMAP server POP server User agent SMTP server SMTP server S ending messages: SMTP server Local file server NFS User agent Sender’s intranet Recipient’s mailhost intranet Reading messages: Local file server Recipient’s mailhost intranet User agent POP NFS protocol User agent IMAP User agent SMTP NFS Posting news articles: NNTP server User agent User agent NNTP Browsing/reading articles: NNTP server NNTP server NNTP server NNTP server NNTP server NNTP server NNTP server User agent User agent NNTP Distributed Systems, Edition 3: Chapter 2 Solutions 3 Last updated: 9 October 2000 6:12 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 2.2 For the applications discussed in Exercise 2.1 state how the servers cooperate in providing a service. 2.2 Ans. Web : Web servers cooperate with Proxy servers to minimize network traffic and latency. Responsibility for consistency is taken by the proxy servers - they check the modification dates of pages frequently with the originating web server. Mail : SMTP servers do not necessarily hold mail delivery routing tables to all destinations. Instead, they simply route messages addressed to unknown destinations to another server that is likely to have the relevant tables. Netnews : All NNTP servers cooperate in the manner described above to provide the newsfeed mechanism. 2.3 How do the applications discussed in Exercise 2.1 involve the partitioning and/or replication (or caching) of data amongst servers? 2.3 Ans. Web : Web page masters are held in a file system at a single server. The information on the web as a whole is therefore partitioned amongst many web servers. Replication is not a part of the web protocols, but a heavily-used web site may provide several servers with identical copies of the relevant file system using one of the well-known means for replicating slowly- changing data (Chapter 14). HTTP requests can be multiplexed amongst the identical servers using the (fairly basic) DNS load sharing mechanism described on page 169. In addition, web proxy servers support replication through the use of cached replicas of recently-used pages and browsers support replication by maintaining a local cache of recently accessed pages. Mail : Messages are stored only at their destinations. That is, the mail service is based mainly on partitioning, although a message to multiple recipients is replicated at several destinations. Netnews : Each group is replicated only at sites requiring it. 2.4 A search engine is a web server that responds to client requests to search in its stored indexes and (concurrently) runs several web crawler tasks to build and update the indexes. What are the requirements for synchronization between these concurrent activities? 2.4 Ans. The crawler tasks could build partial indexes to new pages incrementally, then merge them with the active index (including deleting invalid references). This merging operation could be done on an off-line copy. Finally, the environment for processing client requests is changed to access the new index. The latter might need some concurrency control, but in principle it is just a change to one reference to the index which should be atomic. 2.5 Suggest some applications for the peer process model, distinguishing between cases when the state of all peers needs to be identical and cases that demand less consistency. 2.5 Ans. Cooperative work (groupware) applications that provide a peer process near to each user. Applications that need to present all users with identical state - shared whiteboard, shared view of a textual discussion Less consistency: where a group of users are working on a shared document, but different users access different parts or perhaps one user locks part of the document and the others are shown the new version when it is ready. Some services are effectively groups of peer processes to provide availability or fault tolerance. If they partition data then they don’t need to keep consistent at all. If they replicate then they do. Distributed Systems, Edition 3: Chapter 2 Solutions 4 Last updated: 9 October 2000 6:12 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 2.6 List the types of local resource that are vulnerable to an attack by an untrusted program that is downloaded from a remote site and run in a local computer. 2.6 Ans. Objects in the file system e.g. files, directories can be read/written/created/deleted using the rights of the local user who runs the program. Network communication - the program might attempt to create sockets, connect to them, send messages etc. Access to printers. It may also impersonate the user in various ways, for example, sending/receiving email 2.7 Give some examples of applications where the use of mobile code is beneficial. 2.7 Ans. Doing computation close to the user, as in Applets example Enhancing browser- as described on page 70 e.g. to allow server initiated communication. Cases where objects are sent to a process and the code is required to make them usable. (e.g. as in RMI in Chapter 5) 2.8 What factors affect the responsiveness of an application that accesses shared data managed by a server? Describe remedies that are available and discuss their usefulness. 2.8 Ans. When the client accesses a server, it makes an invocation of an operation in a server running in a remote computer. The following can affect responsiveness: 1. server overloaded; 2. latency in exchanging request and reply messages (due to layers of OS and middleware software in client and server); 3. load on network. The use of caching helps with all of the above problems. In particular client caching reduces all of them. Proxy server caches help with (1). Replication of the service also helps with 1. The use of lightweight communication protocols helps with (2). 2.9 Distinguish between buffering and caching. 2.9 Ans. Buffering: a technique for storing data transmitted from a sending process to a receiving process in local memory or secondary (disk) storage until the receiving process is ready to consume it. For example, when reading data from a file or transmitting messages through a network, it is beneficial to handle it in large blocks. The blocks are held in buffer storage in the receiving process’ memory space. The buffer is released when the data has been consumed by the process. Caching: a technique for optimizing access to remote data objects by holding a copy of them in local memory or secondary (disk) storage. Accesses to parts of the remote object are translated into accesses to the corresponding parts of the local copy. Unlike buffering, the local copy may be retained as long as there is local memory available to hold it. A cache management algorithm and a release strategy are needed to manage the use of the memory allocated to the cache. (If we interpret the word ‘remote’ in the sense of ‘further from the processor’, then this definition is valid not only for client caches in distributed systems but also for disk block caches in operating systems and processor caches in cpu chips.) [...]... the names and types of the instance variables.Then serialize each instance variable recursively Distributed Systems, Edition 3: Chapter 4 Solutions Last updated: 21 July 2000 11:41 am ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 3 serialize(Object o) { c = class(o); class_handle = get_handle(c); if (class_handle==null) // write class information and define class_handle; write class_handle write... attacks Denial of service: flooding a publicly-available service with irrelevant messages Distributed Systems, Edition 3: Chapter 2 Solutions Last updated: 9 October 2000 6:12 pm ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 7 Distributed Systems: Concepts and Design Edition 3 By George Coulouris, Jean Dollimore and Tim Kindberg Addison-Wesley, ©Pearson Education 2001 Chapter 3 3.1 Exercise Solutions... Dollimore and Tim Kindberg 2000 7 Distributed Systems: Concepts and Design Edition 3 By George Coulouris, Jean Dollimore and Tim Kindberg Addison-Wesley, ©Pearson Education 2001 Chapter 4 Exercise Solutions 4.1 Is it conceivably useful for a port to have several receivers? 4.1 Ans If several processes share a port, then it must be possible for all of the messages that arrive on that port to be received and. .. constructor from its parameter types and to create a new instance of an object from the constructor and the argument values 4.11 Ans Whenever a handle definition is read, i.e a class_info, handle correspondence or an object, handle correspondence, store the pair by method map When a handle is read look it up to find the corresponding class or object Distributed Systems, Edition 3: Chapter 4 Solutions... Jean Dollimore and Tim Kindberg 2000 11 Distributed Systems: Concepts and Design Edition 3 By George Coulouris, Jean Dollimore and Tim Kindberg Addison-Wesley, ©Pearson Education 2001 Chapter 5 5.1 Exercise Solutions The Election interface provides two remote methods: vote: with two parameters through which the client supplies the name of a candidate (a string) and the ‘voter’s number’ (an integer used... be handled by the OS, or by a standard daemon process The overheads of examining the message, parsing it and deciding whether it need be acted upon are incurred by every host on the network, whereas only a small number are likely locations for a given resource Despite this, Distributed Systems, Edition 3: Chapter 3 Solutions Last updated: 21 February 2001 3:29 pm ©George Coulouris, Jean Dollimore and. .. (class_handle==null) // write class information and define class_handle; write class_handle write number (n), name and class of each instance variable object_handle = get_handle(o); if (object_handle==null) { define object_handle; for (iv = 0 to n-1) if (primitive(iv) ) write iv else serialize( iv) } write object_handle } To describe the serialized form that your algorithm would produce when serializing an instance... socket and then send n UDP datagram messages to the Distributed Systems, Edition 3: Chapter 4 Solutions Last updated: 21 July 2000 11:41 am ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 1 server Message i should contain the integer i in the first four bytes and the character ‘*’ in the remaining l-4 bytes It does not attempt to receive any messages Take a copy of the program in Figure 4.4 and. .. 4.18 Describe the ways in which the request-reply protocol masks the heterogeneity of operating systems and of computer networks Distributed Systems, Edition 3: Chapter 4 Solutions Last updated: 21 July 2000 11:41 am ©George Coulouris, Jean Dollimore and Tim Kindberg 2000 8 4.18 Ans (i) Different operating systems may provide a variety of different interfaces to the communication protocols These interfaces... converting it into a standard big-endian form before transmission Discuss the advantages and disadvantages of this method when compared with CORBA’s CDR 4.7 Ans The XDR method which uses a standard form is inefficient when communication takes place between pairs of similar computers whose byte orderings differ from the standard It is efficient in networks in which the byteDistributed Systems, Edition 3: . queuing scheme). Distributed Systems: Concepts and Design Edition 3 By George Coulouris, Jean Dollimore and Tim Kindberg Addison-Wesley, ©Pearson Education 2001 Distributed Systems, Edition. valid not only for client caches in distributed systems but also for disk block caches in operating systems and processor caches in cpu chips.) Distributed Systems, Edition 3: Chapter 2 Solutions. addresses and network id for all hosts on the local network (connected set of Ethernets accessible from the switch). It does this by ‘learning’ the host Distributed Systems: Concepts and Design